The ERG is a small response that is generated by the retina in
response to light. The response recorded provides information about
how well the photo receptors are changing light into a signal that
passes to the optic nerve.
Rod and cone function can be differentiated, as well as and inner and
outer retinal function, by changing wavelength, intensity and duration
of a flash stimuli under different states of dark and light adaptation.
The recorded ERG waveform is a summation of retinal potentials from
all retinal areas, which have different polarity, latency and amplitude.
An ERG can also be recorded in response to an alternating
checkerboard pattern (pattern ERG). This provides information about
the part of the retina that images are focused on (macula).